Frame synthesis.
To generate the intermediate range image, following the coarse-to-fine architecture, we design the frame synthesis networks consisting of two cascaded U-Nets [1]} with the same architecture, i.e., the frame pre-synthesis network and the frame refinement network respectively in Fig. REF . This is motivated by the fact that, benefited from a skip connection between the encoder and decoder, U-Net is well suitable for the operation at pixel level [2]}. To reduce the computation complexity, the cascade U-Nets are modified with less depths, which contain a six-layered encoder and five-layered decoder. We use \(3\times 3\)  kernels for all convolutional layers, followed with Leaky ReLu (\(\alpha \)  \(=\)  0.1). It should be noted that as illustrated previously, batch normalization is not adopted.
